By utilizing two quadrature polarization states (x and y) of light, dual-polarized system can simultaneously transmit two independent signals (h and v, which are also referred to as first information source and second information source) within the same bandwidth, to thereby enhance transmission efficiency of the channel by two times. But there may be many linear transmission distortions generated in the optical fiber channel due, for instance, to chromatic dispersion (CD), polarization-mode dispersion (PMD), polarization dependent loss (PDL) and limitations of optical/electrical filter bandwidth. Besides, the optical fiber channel changes the polarization state of the input signal, and thus leading to crosstalk between two branches of signals, that is to say, the polarization state x simultaneously contains the information of h and v, while the polarization state y also simultaneously contains the information of h and v. The polarization diversity coherent receiver simultaneously recovers the signals (including the cophase component and the quadrature component) of the x and y polarization states. The linearity of the channel remains, so electrical equalization is suitable to de-multiplex and compensate various distortions in the channel. Because the change of the polarization states by the channel and other distortions (such as those caused by PMD) are time varying, electrical equalization of the receiver must be a dynamic compensation system that tracks the change in real time. However, the method of blind equalization is usually employed in the optical communication system due to the lack of a training signal. Such an example is provided in documents OFC07 paper OTuA1 “Transmission of 42.8 Gbit/s Polarization Multiplexed NRZ-QPSK over 6400 km of Standard Fiber with no Optical Dispersion Compensation”.
FIG. 1 is a block diagram showing a dual-polarized coherent receiver of the prior art. As shown in FIG. 1, the input optical signal 101 of the dual-polarized optical coherent receiver comprises two polarization components. The input optical signal 101 is separated by means of a polarization beam splitter 103 into two components of the x direction and the y direction to be respectively connected to the first input port of an x-branch 90° optical mixer 105 and the first input port of a y-branch 90° optical mixer 106. A receiving end local oscillator 102 (local laser) is connected to a 50:50 coupler 104, and its output is respectively outputted to the second input port of the x-branch 90° optical mixer 105 and the second input port of the y-branch 90° optical mixer 106. Two outputs of the x-branch 90° optical mixer 105 are respectively outputted to twin photoelectric detectors 107 and 108, and two outputs of the y-branch 90° optical mixer 106 are respectively outputted to twin photoelectric detectors 109 and 110. Outputs of the twin photoelectric detectors 107, 108, 109 and 110 are respectively connected to analog-to-digital converters 111, 112, 113 and 114. These constitute the front-end processing section of the dual-polarized coherent receiver. The front-end processing section of the dual-polarized coherent receiver separates and converts the optical signal 101 into baseband digital signals of two directions, ie. Ix+jQx 115 (also referred to as the first component signal) and Iy+jQy 116 (also referred to as the second component signal), where Ix is the cophase component in the x direction, Qx is the quadrature component in the x direction, Iy is the cophase component in the y direction, and Qy is the quadrature component in the y direction.
As should be noted, the above illustration to the front-end processing section of the dual-polarized coherent receiver is only exemplary in nature. It can also be implemented by self-coherent detecting devices, field identification processing devices or other front-end processing sections of other structures well known to persons skilled in the art, and these all fall within the scope of the front-end processing section according to the present invention.
The baseband digital signals of two directions Ix+jQx 115 and Iy+jQy 116 are connected to the downstream digital signal processing unit 141 for completion of data recover. The equalizer of the digital signal processing unit of the dual-polarized coherent receiver is of a disc-shaped structure. The x-branch baseband digital signal Ix+jQx 115 is inputted to filters Hxx 117 and Hxy 119.
The filter Hxx 117 filters the baseband digital signal 115 to obtain a first information source component of the dual-polarized system.
The filter Hxy 119 filters the baseband digital signal 115 to obtain a second information source component of the dual-polarized system.
The y-branch baseband digital signal Iy+jQy 116 is inputted to filters Hyx 118 and Hyy 120.
The filter Hyx 118 filters the baseband digital signal 116 to obtain another component of the first information source of the dual-polarized system.
The filter Hyy 120 filters the baseband digital signal 116 to obtain another component of the second information source of the dual-polarized system.
Outputs of the filters Hxx 117 and Hyx 118 are inputted to an x-branch adder 121, and an x-branch equalized signal 131 outputted from the adder 121 is inputted to an x-branch phase recovering unit 123 to obtain a signal 133 whose phase has been recovered. The signal 133 is inputted to an x-branch data recovering unit 125 to obtain a recovered x-branch codebook 135. Outputs of the filters Hxy 119 and Hyy 120 are inputted to a y-branch adder 122, and a y-branch equalized signal 132 outputted from the adder 122 is inputted to a y-branch phase recovering unit 124 to obtain a signal 134 whose phase has been recovered. The signal 134 is inputted to a y-branch data recovering unit 126 to obtain a recovered y-branch codebook 136.
To facilitate description, the circuit formed by the filter 117, the filter 118, the adder 121, the phase recovering unit 123, the data recovering unit 125 as well as a blind error estimating unit 127 and a filter coefficient updating unit 129 to be discussed later is called the first branch, and the circuit formed by the filter 119, the filter 120, the adder 122, the phase recovering unit 124, the data recovering unit 126 as well as a blind error estimating unit 128 and a filter coefficient updating unit 130 to be discussed later is called the second branch.
The x-branch equalized signal 131 is further inputted to the x-branch blind error estimating unit 127, so as to estimate the equalization error. The x-branch baseband digital signal Ix+jQx 115, the y-branch baseband digital signal Iy+jQy 116, the x-branch equalized signal 131, and a blind equalization error estimating signal outputted from the x-branch blind error estimating unit 127 are inputted together to the x-branch filter coefficient updating unit 129 to obtain updated filter Hxx coefficient 137 and filter Hyx coefficient 138. The updated filter Hxx coefficient 137 and filter Hyx coefficient 138 are respectively inputted to the filter Hxx 117 and the filter Hyx 118 for update of coefficients and completion of tracking the system change.
Similarly, the y-branch equalized signal 132 is further inputted to the y-branch blind error estimating unit 128, so as to estimate the equalization error. The x-branch baseband digital signal Ix+jQx 115, the y-branch baseband digital signal Iy+jQy 116, the y-branch equalized signal 132, and a blind equalization error estimating signal outputted from the y-branch blind error estimating unit 128 are inputted together to the y-branch filter coefficient updating unit 130 to obtain updated filter Hxy coefficient 139 and filter Hyy coefficient 140. The updated filter Hxy coefficient 139 and filter Hyy coefficient 140 are respectively inputted to the filter Hxy 119 and the filter Hyy 120 for update of coefficients and completion of tracking the system change. In understandable circumstances, the filter Hxx coefficient 137, the filter Hyx coefficient 138, the filter Hxy coefficient 139 and the filter Hyy coefficient 140 are collectively referred to in the present invention as filter coefficients.
As can be seen from the structure mentioned above, the x-branch and the y-branch are identical in structure, and data processing of each is independent of the other. It is easy, in the case of blind equalization, to lead to the degraded status whereby the x-branch and the y-branch converge to the same information source. Addressing this problem, US Patent Publication US2005/0196176A1 proposed by Han Sun et al provides an algorithm for blind equalization of a dual-polarized coherent receiver to prevent two branches of signals from converging to the same information source. The method undergoes three operation modes: 1) blind self-recover procedure, to obtain a branch of converged channel; 2) training mode, to utilize a synchronous training sequence so that two branches converge to different information sources; and 3) blind equalization mode fed back from decision. This method employs the synchronous training sequence to separate the two branches. Although it is possible to provide the training sequence in the initializing process at the beginning, there is nonetheless no available synchronous training sequence during normal operation status of the system. At the same time, it is risky for error diffusion when the method runs in the decision feedback mode.